As I said it does not simply concatenate lines. The lines are joined in a way that has a reasonable chance of producing reasonably formatted code, not simply deleting the line breaks. "Join lines" seems to be the most common term used by editors for the operation, google found Eclipse, IntelliJ, emacs, vim in page one.
Yeah, `_U` would probably be better, maybe not being U is a hangover from when there was another U in the menu, git would know. When removing the other it was a good thing not to change `_U` to do something different and so confuse users. But anyway its now what it is and may induce howls of protest if we change it (maybe next major release the whole accelerators thing can be cleaned up).